Radio Frequency Identification (RFID) is a technology of using radio communication for exchanging information stored in a tag. The RFID technology includes two distinct types: an active type wherein a tag has a built-in battery and power for the operation is supplied internally; and a passive type wherein a tag does not have a built-in battery and operates using power which is supplied by a high frequency wave transmitted from a reader/writer device. The passive type tags can be offered at a relatively lower price than the active type tags and are expected to be useful in a variety of areas including the field of physical distribution. When the UHF band (860 to 960 MHz) is used as the frequency band of RFID, the radio communication area becomes wider than the other frequency band even for the passive type, so that a plurality of tags can be read at once. Accordingly, it becomes possible to read a plurality of tags attached to a number of articles at one time for inspection in the field of physical distribution, for example.
However, as a result of broadening the communication area, the UHF band RFID has an inherent problem that a tag is sometimes read unintendedly. For example, at the time of shipping inspection at a warehouse or the like, a tag attached to an article placed at a position which is so far from an inspection gate that the tag is normally not read is accidentally read by a radio wave reflected at a forklift which passes close to the article. When a plurality of gates are placed side by side, there is another problem that a tag of an article which comes in an adjacent gate is read.
In such a case, an unnecessary tag can be sometimes eliminated by filtering based on an ID stored in a tag. For example, when an ID is composed of some element including logistics types (e.g., pallet or individual) of an article or the like, a tag ID indicative of a pallet tag can be eliminated by knowing type data indicative of the type of an article in advance. Moreover, when unintended tag reading is caused by reflection of a radio wave or the like, there is a known method for periodically executing a plurality of times of detection for a tag attached to an article and eliminating an ID of a tag which is not detected continuously over a predetermined number of times, assuming that the tag was read accidently by reflection (see Patent Document 1: Japanese Patent Application Laid-Open No. 2005-275960).
However, when a target tag and an unnecessary tag belong to the same type of an article, judgment by type data included in a layered ID cannot be used. Moreover, filtering using only continuous detection such as Patent Document 1 can be performed only in extremely limited instances. Although there is another commonly-used measure of physically isolating a reading area with a radio wave adsorption board or the like to eliminate unintended tag reading, this method has a problem that the man-hour of field operation of installation increases drastically. Furthermore, although there is another method of using a phased-array antenna or the like which can change the antenna directivity thereof to find a tag which is actually out of a reading area and identify the tag as an unnecessary tag, a new problem arises that a reading device becomes expensive.